In view of the now, well-established relationship between chlorofluorocarbons (CFC's) released into the atmosphere and the depletion of the earth's ozone layer, considerable attention is being directed to finding effective substitutes for these once widely used compounds. It appears that the worst offenders are the fully halogenated CFC's which contain chlorine. These compounds are relatively unreactive with other compounds in the lower atmosphere and thus are able to diffuse into the stratosphere intact and be decomposed by ultraviolet radiation to form inter alia, chlorine free radicals which readily react with ozone. On the premise that it is the chlorine constituent of the CFC's which ultimately reacts with and destroys the ozone molecules and in the interest of approximating as closely as possible the physical properties of the CFC's already in use, the proposed substitutes in general have been HCFC's containing lesser proportions of chlorine or fluorocarbons containing no chlorine at all. For example, dichlorodifluoromethane, widely used under the trademark Freon 12 as a refrigerant in household refrigerators, in automotive units and in commercial freezers and display cases, has been replaced in many instances by 1,1,1,2-tetrafluoroethane (also known as R-134a) or by chlorodifluoromethane (also known as R-22 or HCFC-22). Because R-134a is not miscible with many commonly used lubricants, mixtures of R-134a and R-22 have been proposed for systems employing lubricants soluble in R-22. See U.S. Pat. No. 5,198,139, Bierschenk et al, in this regard. In the recent past, over 90% of the chlorodifluoromethane and about a third of the dichlorodifluoromethane manufactured was utilized in air-conditioning and refrigeration.
U.S. Pat. No. 3,536,521 to McKinney et al. discloses a method of preventing the adsorption of gases other than water, such as fluorinated hydrocarbons, by the coating of Type A zeolite molecular sieves with silicones such as methyl silicone. McKinney et al. further discloses that the fluorinated hydrocarbons used in refrigerant systems react at active sites on the surface of the molecular sieve with subsequent decomposition into halogen acids which react with the basic structure of the molecular sieve.
U.S. Pat. No. 3,625,866 to Conde discloses a process for preparing composite desiccant materials for applications such as refrigerant drying wherein the pores of the desiccant are less than about 4.9 Angstroms in diameter to permit the inclusion of water molecules and to exclude the larger halogenated hydrocarbon molecules. Conde discloses the use of zeolite 3A which he describes as prepared by replacing at least 65% of the sodium cations in zeolite A with potassium cations by conventional cation exchange techniques to adsorb molecules having critical diameters up to 3 Angstrom units. Conde hardens the zeolite A into an agglomerate optionally with clay, silicates or both and then applies to the surface of the agglomerate a thin coating of diaspore (hydrated alumina) and a clay mineral, wherein the diaspore is the major component. The coated agglomerate is then soaked in an aqueous solution of potassium silicate to impregnate silicate into the agglomerate, dried in air to avoid steaming during calcination, and finally fired to set the binder and activate the molecular sieve. The refrigerant decomposition tests were based on R-22. Conde discloses that clays which may be employed for bonding molecular sieves without substantially altering the adsorptive properties of the molecular sieve are attapulgite, kaolin, sepiolite, palygorskite, kaolinite, plastic ball clays, clays of the attapulgite or kaolin types, bentonite, montrnorillonite, illite, chlorite, and bentonite-type clay.
U.S. Pat. No. 5,347,822 to Cannan et al. discloses the use of a microporous zeolite molecular sieve having the crystal structure of zeolite B, and a framework silica to alumina molar ratio of at least 2.5 for use in refrigeration systems containing R-32. Cannan et al. discloses that the modified form of zeolite B has pore openings small enough to significantly limit the adsorption of R-32 while retaining a large capacity for water adsorption.
With increasing recognition of the seriousness of atmospheric ozone depletion, stricter limitations on the future use of any chlorine-containing refrigerant continue to be imposed. One of the most suitable replacements for R-22 in stationary refrigeration systems is a non-flammable mixture of the HFC compound difluoromethane, also known as R-32, with other halocarbons or halohydrocarbons such as R-134a and R-125 (C.sub.2 HF.sub.5). One such mixture known as R-410a has been proposed and consists of 50% R-32 and 50% R-125. Another proposed mixture (R-407c) consists of 23% R-32, 25% R-125, and 52% R-134a. A significant problem in making this substitution arises from the fact that R-32 is more readily adsorbed than R-22 with zeolite A, commonly employed as an adsorbent-desiccant in the circulating refrigerant stream to protect against freeze-ups and corrosion of the refrigeration unit. Ideally, a purified and dried refrigerant fluid, after having been sealed in a refrigeration unit, would continue to circulate through the compressors, Joule-Thompson nozzles, cooling coils, etc., for the life of the unit without causing any corrosion or freeze-up problems. In practice, however, the system can rarely be so thoroughly sealed or the components so thoroughly dried before sealing to prevent water and other contaminants from entering the sealed system. These contaminating materials must be removed or sequestered to avoid the development of the aforementioned problems. Conventionally, the contaminants are rendered innocuous by adsorption thereof on a suitable adsorbent which is inserted into the sealed system and which is in contact with the circulating refrigerant stream. In the case of halocarbon refrigerants, the contaminants of greatest concern, in addition to water, are attributable to the degradation products of the refrigerant molecules themselves. Halogen acids, notably HCl, can form and cause corrosion. In some instances, the adsorbent composition itself can be a reactant in the chemical reactions which result in the production of corrosive products. Zeolitic molecular sieves generally exhibit this property. Accordingly, in view of the physical and chemical properties of the refrigerant involved, it is necessary to select the particular zeolite adsorbent to minimize harmful reactions. Since essentially all of the active sites of a zeolite are reachable only by molecules which can enter the internal cavities of the crystal structure through its uniform pore system, it is advantageous to employ a zeolite whose pore openings admit water and other small impurity molecules and exclude molecules of the refrigerant. Thus, a commonly used adsorbent for refrigeration systems is a highly exchanged potassium cation form of zeolite A having pore diameters of about 3 Angstroms. The effective pore diameters can be further reduced, to a slight degree, by controlled steaming as disclosed in U.S. Pat. No. 3,506,593, hereby incorporated by reference. A potassium cation exchanged (40%) form of zeolite A, i.e., zeolite 3A, has been found to be quite effective in drying R-134a and R-22, for example.
R-32, however, is both smaller in molecular size and more polar than R-22 by virtue of the substitution of a hydrogen atom for the chlorine atom in chlorodifluoromethane. It is also more reactive than R-22 with constituents in the lower atmosphere and thus, advantageously, is less likely to escape unreacted into the stratosphere. It is, by the same token, more reactive with zeolites, including zeolite 3A, having pores large enough for R-32 to enter. The greater polarity of R-32 also means that the partial blocking of zeolite pores by cation exchange techniques is less effective in excluding the R-32 from the inner cavities of the zeolite crystal structure. Adsorbent aggregates are sought which minimize reactivity with difluoromethane without reducing the capacity of the adsorbent to adsorb water for use as desiccant in closed-cycle refrigeration systems.
When a molecular sieve adsorbs the refrigerant molecule, there is a much greater chance of chemical interaction between the refrigerant and desiccant, and such interaction will result in the chemical destruction of both. By "destruction of both," it is meant that the refrigerant may become decomposed into undesirable products and that the desiccant can lose significant water adsorption capacity. Also, when the refrigerant is adsorbed, it is using up some of the desiccant's capacity for water adsorption. Finally, the composition of a refrigerant blend will be changed if the smaller, more polar refrigerant (like R-32) is selectively adsorbed and removed from the system. All of this is avoided by excluding the refrigerant from the desiccant.